Three million phenytoin (PHT)-treated epileptic Americans manifest some degree of gingival overgrowth. In these lesions, an increase in the dimension of the connective tissue component of the gingivae is always observed, yet the pathogenesis of the condition has remained largely unknown. Data from our laboratory using a feline model indicate that a PHT metabolite, acting alone or in combination with the parent compound, plays an etiologic role. Furthermore, from our in vitro cell culture studies we have determined that the most likely mechanism of action is one of in vivo selection of a phenotypically stable subpopulation of fibroblasts characterized by elevated protein, collagen and glycosaminoglycan synthetic activities, as well as secretion of a largely inactive collagenase. The broad aim of the proposed studies is to elucidate further the roles of PHT metabolites and fibroblast subpopulation selection in the pathogenesis of PHT-induced gingival overgrowth. Specifically, we shall determine the in vitro effect of 3 major PHT metabolites on cultured human and feline gingival fibroblasts, and evaluate the PHT-sensitivity of 30 strains of "normal" human gingival fibroblasts in an attempt to detect genetically susceptible PHT-responder individuals. We shall also derive numerous single-cell clones from mixed parent populations of cells, perform interclone comparisons of synthetic parameters, growth characteristics and cell size distributions, and examine selected clones ultrastructurally to detect heterogeneous clones. Experiments will be performed to ascertain why the collagenase produced by responder cells is inactive, and to determine the chemical nature of the glycosaminoglycans that are produced in excess by these cells. Also in cell culture, we will assay functional parameters of feline gingival fibroblasts before and after systemic administration of PHT. Finally, our heretofore successful breeding endeavors will be continued, specifically crossing the F1 offspring of known responder and nonresponder mongrel cats. Knowledge gained from the proposed experiments may provide the rationale for development of new prophylactic and/or therapeutic approaches in the control of gingival enlargement as well as other similar-appearing and clinically more significant fibrotic-hyperplastic connective tissue lesions such as burn scar, keloid, arthritis, scleroderma, epidermolysis bullosa and systemic lupus erythematosis.